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Dongguan Donggang Electronics Co., Ltd.
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- 2019-06-12-

Daily maintenance of low frequency transformer

1. In order to protect the insulation performance of the high-voltage generator and the insulating oil installed in the machine head, generally do not open the observation window and loosen the surrounding fixing screws to prevent the oil from absorbing moisture or falling dust and reducing the insulation performance.

2. Check whether the peripheral lighting, heat dissipation and dust removal equipment of the transformer are in good condition, and wipe the dust on the transformer body and the porcelain bottle with a clean cloth.

3. Check the high-voltage side load switch of the transformer to ensure flexible operation, good contact, and lubrication treatment of the transmission part.

4. Pull open the high-voltage grounding knife, check that the grounding is in the disconnected position, then close the high-voltage load switch, let the transformer test run, and remove the high-voltage side identification plate. Note that when disconnecting or closing the transformer high-voltage load switch, the site There must be more than two people.

5. When it is necessary to replace the new oil, the local electric power department should be assisted to check the performance of the new oil, and the insulation strength should be no less than 25000 volts / 2.5 mm; and the oil insulation strength in the combined head should be 30,000 volts / 2.5 mm or more.

6. Measure the insulation resistance of the high and low voltage coils of the transformer (ground and phase) with a 2500V shaker. Confirm that the requirements are met. (At room temperature 30 °C, the high voltage side of the 1OKV transformer is greater than 20MΩ, and the low voltage side is greater than 13MΩ. Before the test, it should be connected. Good grounding wire, after the measurement is completed, should be discharged.

7. The high voltage generator or the combined machine head must have a good grounding wire. Always use an ohmmeter to measure whether the outer casing, the console casing and the external grounding wire are conducting, and tighten the grounding bolt.


Frequently asked questions about low frequency transformers

DCR problem

A. DCR is too high. Due to the thin wire diameter of the silicon steel sheet transformer, the number of turns is very large, and the choice of enameled wire becomes an important part. Most US customers prefer to use the AWG line to design and set up DCR specifications and structures. For some reason, we have to use the mm line approximation instead. The bare diameter of the enameled wire in Taiwan is a little bit larger, and the coating film is also a little thicker. Therefore, the DCR will be super high in thousands of laps, and the wire pack will be fat. Another point is that the wire package is fat, neatly wrapped and messy, and the DCR will be different with different line lengths, and the neat and tight winding will be lower.

B. The difference between the two groups of DCR values Some specifications require and around (or separate UI CORE twins) the difference between the DCR or the SPEC requires the twin secondary output to be idling, the voltage of the two should not exceed 0.1V, here In this case, two products that are wound out on the same axis of the same winding machine should be assembled to obtain a good success rate.

Excitation current problem

In general power transformers, the excitation current specification is not very strict, and it refers to the primary current when the secondary side is not loaded. If the excitation current is exceeded, the possible causes are:

a. The number of turns is too small, or the wire diameter is too large

b. Silicon steel sheet is not assembled

c. Silicon steel sheet material is not good enough

d. Silicon steel sheet is too thick

e. Instrument error

f. If the input conditions are correct, if the excitation current exceeds a lot, check if the coil is short-circuited.

The loss is too high

Our wattmeter reading on the test board exceeded the specifications (both copper and iron loss) because the winding was completed, and it was customary to improve from the iron loss:

a. The silicon steel sheet is not installed (the edges are not uniform, not tightened)

b. Improve the core material

c. Select a thin silicon steel sheet.

Voltage adjustment rate is too large

ΔU%=(U20-U2)/U20 U20 : No-load output voltage U2: The load output voltage is related to the copper loss, iron loss, and number of turns in the design.

Temperature rise

It can be calculated by the resistance method (234.5+T1)/R1 = (234.5+T2)/R2 6. HI-POT and IR (insulation resistance) The HI-POT test is a requirement for the transformer to be fully tested for 100 points. Many customers also require each The shipment must be accompanied by a statement of the HI-POT full test.

Causes of HI-POT failure:

a. Lead wire contact between two windings or too close

b. Short circuit between PIN bridges

c. Insulating tape is not completely covered or the number of layers is too small

d. Insufficient distance between winding and CORE

e. CORE rubs BOBBIN or CASE during assembly. When the cutoff current is 3mA on SPEC, do not test HI-POT with 2mA or 1mA. 2mA is more strict than 3mA.


In principle, the transformer of the silicon steel sheet is not subjected to vacuum impregnation. Excessive vacuum pressure will cause VAR water to penetrate between CORE, causing gaps and reducing power


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